Switching regulators regulate voltage across a load connected to its output by varying the ON-OFF times of switching elements so that power is transmitted through the switching elements into energy storage elements. The energy storage elements then supply this power to the load. Switching regulators vary the ON-OFF times of the switching elements, in part, responsive to a clock signal generated by an oscillator. In a manner to be discussed in greater detail hereinbelow, the noise at the output of the switching regulator is dependent on the switching frequency of the clock signal, which hereinafter also will be referred to as the operating frequency of the switching regulator.
A switching regulator introduces electromagnetic noise to an electronic application. While various techniques can be used in reducing the radiated and common-mode noise, differential-mode noise can be neither shielded nor snubbed. Instead, it directly is passed along the power distribution path. A typical fixed frequency switching regulator has a differential-mode noise spectrum as shown in FIG. 1, with high peaks of undesirable energy concentrated at the switching frequency (fS) and its harmonics.
In general, two kinds of techniques are available in reducing the differential-mode noise: filtering and spectrum spreading. Filtering attenuates noise by adding additional components, which either have to conduct full supply current or support full supply voltage. To accommodate such high power transmission, those additional components can be physically large. In contrast, spectrum spreading deals with the noise problem from the clock source. Without the use of additional power components to conduct high power, spectrum spreading modulates the instantaneous operating frequency of a switching regulator over a span of switching frequencies, attenuating the peak noise amplitude by distributing the energy across the span of switching frequencies. This reduces the conducted interference of the switching regulator with its downstream devices, often resulting in better noise reduction than filtering.
There have been different methods for spectrum spreading to reduce differential-mode noise. Depending on how the switching frequency is modulated over time, those existing methods can be sorted into two major categories of frequency modulation: sinusoidal and linear frequency modulation.
Early research on frequency modulation for switching-mode power supplies used sinusoidal modulation, in which the operating frequency is modulated in accordance with a sinusoidal frequency modulation waveform (see, e.g., FIG. 2A). Sinusoidal frequency modulation may include (1) sequential sinusoidal frequency modulation in which the switching frequency increases and decreases with time along a smooth or step-wise continuous sinusoidal curve, and (2) pseudo-random sinusoidal frequency modulation in which the switching frequency “hops” among different frequencies in a pseudo-random fashion in which the values of the switching frequencies over a period of time, if sorted in numerical order, form approximately a sinusoidal curve. Since the time derivative of a sinusoidal waveform is greatest at its middle points but equals zero at the peaks and valleys (which correspond to the maximum and minimum frequencies, respectively), the resulting noise spectrum has peaks or “horns” at the frequency extremes, thereby affecting the efficiency of noise reduction. When the frequency of a switching regulator is modulated in accordance with a sinusoidal frequency modulation waveform, the resulting differential-mode noise spectrum across the output capacitor of a switching regulator illustratively is depicted in FIG. 2B.
Linear frequency modulation modulates the switching frequency in accordance with a linear frequency modulation waveform such as by (1) sequential linear frequency modulation in which the switching frequency increases and decreases with time along a smooth or step-wise continuous linear curve, and (2) pseudo-random linear frequency modulation in which the switching frequency “hops” among different frequencies in a pseudo-random fashion in which the values of the switching frequencies over a period of time, if sorted in numerical order, form a straight line between the minimum and maximum switching frequencies. Although linear frequency modulation yields better noise reduction than the sinusoidal method, it still suffers from higher noise amplitudes (or “horns”) at frequency extremes. See, e.g., FIGS. 2C-D, which respectively provide an illustrative linear frequency modulation waveform and the resulting differential-mode noise spectrum across the output capacitor of a switching regulator when the regulator's switching frequency is modulated in accordance with a linear frequency modulation waveform.
U.S. Pat. No. 5,488,627 to Hardin et al. (“the Hardin patent”) and “Spread Spectrum Clock Generation for the Reduction of Radiated Emissions” by Hardin et al., Proceedings of IEEE EMC Conference (1994) (“the Hardin article”) describe a third frequency modulation waveform (“the Hardin frequency modulation waveform”) that was developed mainly to reduce radiated noise, rather than differential-mode noise. However, when the frequency of a switching regulator is modulated in accordance with a waveform similar to the Hardin modulation waveform, the “horns” in the differential-mode noise amplitude at the frequency extremes also is reduced.
FIG. 2E depicts the Hardin frequency modulation waveform (as provided in the Hardin article). When the switching frequency is modulated in accordance with the Hardin frequency modulation waveform, the illustrative radiated noise spectrum shown in FIG. 2F (as provided in the Hardin article) may be generated at the output of a clock that outputs a simple periodic rectangular waveform with a constant amplitude. In contrast, FIG. 3 provides an illustrative differential-mode noise spectrum generated at the output of a switching regulator when its switching frequency is modulated in accordance with a waveform similar to the Hardin frequency modulation waveform. FIG. 3 shows that a waveform similar to the Hardin modulation waveform reduces the “horns” in the differential-mode noise spectrum and may reduce the maximum noise amplitude as compared to that resulting from fixed frequency operation, linear frequency modulation, and sinusoidal frequency modulation. The illustrative noise spectra provided in FIGS. 2B, 2D and 3 are generated using the same power converter.
Unlike the radiated noise spectra shown in FIG. 2F at the output of a clock that generates a simple periodic rectangular waveform having a constant amplitude, the differential-mode noise spectrum at the output of a switching regulator develops a tilted spectral envelope when any of the above-described modulation waveforms are employed in spread spectrum frequency modulation. For example, as illustrated in FIG. 3, the differential-mode noise spectrum across the output capacitor of a switching regulator when the switching frequency is modulated in accordance with a waveform similar to the Hardin frequency modulation waveform disadvantageously tilts from the minimum switching frequency toward the maximum switching frequency at the top of spectral envelope 15 (i.e., spectral ceiling 17).
In view of the foregoing, it would be desirable to be able to provide methods and circuits for spread spectrum frequency modulation that reduce the maximum noise amplitude at the output of a switching regulator by reducing, if not eliminating, the tilt of the spectral noise envelope.
It also would be desirable to be able to provide methods and circuits for spread spectrum frequency modulation that reduce the maximum noise amplitude at the output of a switching regulator by reducing, if not eliminating, the “horns” at the extremes of the frequency modulation span.